The invention relates to cellular radio systems and, in particular, to estimation of speed of a mobile in digital radio systems such as TDMA based systems.
Cellular telephony is a type of short-wave analog or digital transmission in which a subscriber has a wireless connection from a mobile telephone to a relatively nearby base station. The base station""s span of coverage is called a cell. Typically, cellular telephone service is available in urban areas and along major highways but may also be available in some suburban and rural areas. As the cellular telephone user moves from one cell or area of coverage to another, the telephone is effectively passed on to the local cell base station. This passing from base station to base station is called a handoff.
AMPS (Advanced Mobile Phone Service) is a standard system for analog signal cellular telephone service in the United States and is also used in other countries. Time Division Multiple Access (TDMA) is digital transmission technology which allows a number of users to access a single radio frequency channel without interference by allocating unique time slots to each user within each channel. The TDMA digital transmission scheme multiplexes multiple users over a single channel. The current North American TDMA (NA-TDMA) standard for cellular divides a single channel into six time slots, with each user using two slots, providing three times the capacity of AMPS. Each caller is assigned specific time slots for transmission. One xe2x80x9cframexe2x80x9d includes six slots and has a 40 ms duration. A xe2x80x9cslot burstxe2x80x9d lasts 20 ms and includes only three slots. NA-TDMA was first specified as a standard in Electronics Industries Association/Telecommunication Industries Association (EIA/TIA) Interim Standard 54 (IS-54). IS-136 is an evolved version of IS-54.
To cope with the rapidly increasing demand for mobile and personal communications, hierarchical cellular systems, which have multiple layer cellular cells are being deployed in dense urban areas. Specifically, a two-layer hierarchical cellular system consists of microcells overlaid with macrocells, where a macrocell is the union of many microcells. Thus slow-moving mobiles are assigned to microcells and fast-moving mobiles are assigned to macrocells. This approach has an objective of decreasing handoff rate for fast moving mobiles. Hence, a reliable mobile speed estimator is desirable. Benefits of decreasing handoff rate include an increase in capacity for the system and a decrease in the number of dropped calls. As well, voice quality is improved due to reduction of the number of times the voice is muted for handoff.
In the literature, there are a few methods of mobile speed estimation which have been published. Specifically, in A. Sampath and J. M. Holtzman, xe2x80x9cEstimation of Maximum Doppler Frequency for Handoff Decisions,xe2x80x9d Proceedings of IEEE Vehicular Technology Conference, pp.859-862, 1993, the estimation of maximum Doppler frequency can be used to estimate mobile speed for handoff decisions. This approach fails if the spectrum of a fading process has many local maxima. The maximum Doppler frequency can not be easily determined in this case due to different levels of energy at these frequencies.
In K. Kawabata, T. Nakamura, and E. Fukuda, xe2x80x9cEstimating velocity using diversity reception,xe2x80x9d Proceedings of IEEE Vehicular Technology Conference, vol.1, pp.371-374, 1994 and U.S. Pat. No. 5,585,805 issued Dec. 17, 1996 to Tetsuyoshi Takenaka et al. a diversity switching number is used to estimate mobile speed. It is pointed out in T. L. Doumi and J. G. Gardiner, xe2x80x9cUse of base station antenna diversity for mobile speed estimation,xe2x80x9d Electronics Letters, vol.30, no.22, pp.1835-1836, 1994 that this method is highly dependent on the fading distribution statistical properties (Rayleigh fading, Rician fading, etc.) and since it is not easy to know the distribution in advance, this method is not very reliable in practice.
In L. Wang, M. Silventoinen and Z. Honkasalo, xe2x80x9cA new algorithm for estimating mobile speed at the TDMA-based cellular system,xe2x80x9d Proceedings of IEEE Vehicular Technology Conference, pp.1145-1149, 1996, two methods use deviation of received signal strength to estimate mobile speed for Global System for Mobile communication (GSM) radios. Unfortunately, the first method may fail when applied to a signal on a dispersive channel, i.e., in the presence of intersymbol interference. The second method uses pattern recognition to overcome the limitations of the first method on dispersive channels but leads to high computational complexity and may not be reliable. This method can also be applied for IS-136 radios.
In M. Hellebrandt, R. Mathar, and M. Scheibenbogen, xe2x80x9cEstimating Position and Velocity of Mobiles in A Cellular Radio Network,xe2x80x9d IEEE Transaction on Vehicular Technology, vol.46, pp.65-71, 1997, multiple base stations and multidimensional scaling are used to estimate mobile speed. This method may be very expensive in practice.
In W. C. Y. Lee, Mobile Communications Engineering, McGraw-Hill, 1982 (the contents of which are incorporated herein by reference), the level crossing rate (LCR) is used to estimate mobile speed for AMPS radios, unfortunately, it is not reliable for TDMA radios.
U.S. Pat. No. 5,787,348 issued Jul. 28, 1998 to William Willey et al. uses received signal strength and level crossing rate to measure a mobile""s speed. Unfortunately, fluctuations in power of the received signal affect the accuracy of result. As well, this method may not be effective in TDMA due to the inherent 50 Hz interference from the 20 ms slot burst time.
Paul Crichton et al. disclose in U.S. Pat. No. 5,722,072 issued Feb. 24, 1998 a method for determining a handover for a mobile station in a multicellular communication system. Again, received signal strength and level crossing rate are employed. In this method, however, several neighbour cells report to a serving cell which performs the handover determination. Generally, it is expensive to receive reports from several cells.
In U.S. Pat. No. 5,686,925 issued Nov. 11, 1997 to Maeda et al. Global Positioning System (GPS) information is used to determine the speed of a mobile. A disadvantage of this method is a requirement for lines of sight to GPS satellites, which is particularly problematic in urban centres. Although adding GPS circuitry to new mobile handsets may be feasible, supplementing existing handsets with GPS capabilities may be impractical.
Overall, it is necessary to obtain a more reliable algorithm to estimate mobile speed for TDMA-based radios, if handoff reliability in multi-layer hierarchical cells is to be increased.
The method of the subject invention provides mobile speed estimation. A power signal is derived from samples of a received signal. By comparing an estimate of an offset autocorrelation of the power signal to a threshold, the speed of the mobile may be estimated as xe2x80x9cfastxe2x80x9d or xe2x80x9cslowxe2x80x9d. (An xe2x80x9coffsetxe2x80x9d autocorrelation is an autocorrelation where the signal is correlated with a version of itself which has been offset in time.)
In accordance with an aspect of the present invention there is provided a method for estimating a speed of a mobile communication apparatus in a digital mobile communication network. The method includes receiving samples of a signal from the mobile communication apparatus and deriving a power signal from the samples. The method further includes estimating an offset autocorrelation value of the power signal by correlating said power signal with a version of itself offset in time. The offset autocorrelation value is representative of the speed of the mobile communication apparatus.
In accordance with another aspect of the present invention there is provided a computer software medium, for providing program control for a manager of a system which carries out the method of the present invention.
In accordance with a further aspect of the present invention there is provided a speed estimation apparatus for carrying out the method of the present invention.
In accordance with a still further aspect of the present invention there is provided a base station for mobile communications including at least one antenna for receiving a signal from a mobile communication apparatus and at least one analog to digital converter for creating samples of said signal from said mobile communication apparatus. The base station also includes a system controller, a phone service interface and at least one speed estimation apparatus operable to carry out the method of the present invention.
In accordance with another aspect of the present invention there is provided a system for mobile communication including a mobile communication apparatus and a base station including a speed estimation apparatus operable to carry out the method of the present invention.